There is room for improvement
in the functionality of the spot-scanning technique developed at PSI.
Therefore, in tandem with the increase in medical uses for proton
therapy, the gantry and its beam guiding and application techniques
are being optimized. In particular the scanning speed and the sensitivity
of the dynamic beam to organ motion (for example from breathing)
during the treatment are being optimized.
With the current system the target volume (tumor plus safety margins)
is only checked once during each radiation fraction. Due to this technical
limitation only tumors which don’t move, such as those in the
head and neck, spine and pelvis can be treated with optimal precision.
For an expansion of the medical indications to lung and mammary carcinomas
the entire target volume must be scanned 10-20 times faster. This should
be repeatable within the same radiation period.
The new Gantry 2 features ultrafast scanning with
a second magnetic axis which is vertical and located in the gap
before the 90° magnet. Further efficiencies are achieved by guiding
the intensity of the beam directly with the ion source from the dedicated
COMET accelerator by means of a vertical deflector-plate. In the new
PROSCAN facility it is possible to apply the dose with more precision
as the energy of the beam can be dynamically varied with laminated
magnets. Tests have shown that it will be possible to scan a half liter tumor mass in about 7 to 10 seconds..
To fulfil clinical requests,
the design of the new gantry makes it possible to radiate tumors
from nearly all sides. Incorporated in the new design is a floor around
the patient table ensuring that during the treatment, the patient
is always accessible to medical staff.
The goal of all these developments
is to increase radiation precision of even moving tumors so that
the PSI spot-scanning method becomes a viable alternative to the passive
scattering-foil technique also for moving tumors. The compact gantry design could then replace
the giant 10-12 m diameter gantries currently in construction and operation.